The present invention relates generally to hard disc drives, hard disc drive components and other electrical components having a more uniform and predictable, and in some cases modified, resonance spectrum. Particularly, it relates to the structure, construction and arrangement of hard disc drive components or other electrical components to obtain a more uniform and predictable and otherwise improved resonance spectrum.
Computers commonly use disc drives for memory storage purposes. Disc drives include a stack of one or more magnetic discs that rotate and are accessed using a head or read-write transducer. Typically, a high speed motor such as a spindle motor is used to rotate the discs. Voice coil motors are typically used in actuator assemblies to move the heads over the discs.
In many electrically motorized hard disc drive applications, significant mechanical vibration and acoustic noise is generated from both the mechanical and magnetic sources. Mechanical sources include, but are not limited to, things such as static and/or dynamic imbalance of the rotating parts, bearing elasticity and imperfections, windage, and other mechanical means of creating fluctuating forces. In an electric motor, magnetic sources include such things as the magnetostriction from commutation of the current in the electric coils, magnetic force imbalance from arrangement of the poles, slots and coils, and magnetic force imbalance due to eccentricity of the rotor and/or the stator. The vibration from mechanical and magnetic sources usually has an adverse effect on the performance of the motorized spindle. In hard disc drive applications, motor vibration creates undesirable acoustic noise, angular speed variations and data-track mis-registration. It is therefore desirable to reduce the sources of vibration as much as possible.
In hard disc drive applications, it is desirable to have a drive that has a predictable system-wide resonance. The various components in a hard disc drive have their own unique resonance spectrum when the disc drive is in operation. The combination of these resonance spectrums define the system wide resonance spectrum of the hard disc drive. Components such as voice coil motors and spindle motors have subcomponents which also have their unique resonance spectrum. The combination of the resonance spectrums of the motor subcomponents define the system wide resonance for the motor. Sometimes a particular frequency of vibration in one part can couple with the resonate frequency of another part creating a node of energy that is undesirable. As an example a motor bearing may have a defect frequency at 1250 hertz which may excite a resonate frequency of the motor bracket causing a system wide vibration of the motor assembly. Therefore it is desirable to tune the motor so that points of excitation can be manipulated to avoid this excitation phenomena. It is also common that different manufacturers install the various components in the hard disc drive. These variations in system wide resonance must be accounted for in the manufacturing process. A large range of variance in system wide resonance, is a limiting factor in designing servo control logic to drive the heads over the data, in turn limiting the ability of the heads to track the repeatable runout of the media as it spins in hard disc drives.
There is a need for design features and manufacturing techniques that serve to reduce the variance in system wide resonance and obtain a more predictable and uniform system-wide resonance for a hard disc drive and hard disc drive components. Therefore, the present invention provides a method to obtain predictable and uniform system wide resonance as much as possible and to tune the frequency of resonance to enable reductions in sympathetic system wide resonances, thereby leading to lower vibration and noise. The present invention also provides a method of enclosing components of the hard disc drive to obtain a predictable and uniform system wide resonance and to reduce both mechanical and magnetic vibration and noise.
Methods to enclose components of the hard disc drive with a synthetic resin have been suggested, but have not been used to obtain more uniform and predictable system-wide resonance. Prior art methods to mold a synthetic material enclosing hard disc drive components fail to produce a predictable system wide resonance due to several factors. First, the plastic material that is used to enclose the components has variations from lot to lot. In particular, the plastic may vary in viscosity by 60 percent from lot to lot. The molecular weight of the polymer molecules also begins to vary as heat is applied, causing the polymer molecules to become smaller, which causes variations in the viscosity of the molten polymer and stiffness of the solidified polymer. Second, the polymers exhibit non-newtonian rheology and the density of the polymer inside a mold cavity is not uniform. Third, in the past, it has not been possible to control process variables to obtain a relatively uniform volume of polymer, orientation of the polymer as it enters the mold cavity, nor ensure a uniform rate of crystallization as the polymer solidifies.
One example of an overmolded stator and a method of manufacturing such a stator is shown in U.S. Pat. No. 6,075,304 (Nakatsuka) (incorporated herein by reference). Referring to FIGS. 6 and 7 of this patent, a stator 11 is encapsulated with an overmold 12. The patent discloses that the injection speed of the polymer should be more than twice the injection speed of a standard injection molding process. The patent also discloses a molding tool capable of pressure dampening to reduce rapid pressure increase. However, this patent does not teach how to obtain predictable and uniform resonance for a hard disc drive or a hard disc drive components.
An example of a spindle motor is shown in U.S. Pat. No. 5,694,268 (Dunfield et al.) (incorporated herein by reference). Referring to FIGS. 7 and 8 of this patent, a stator 200 of the spindle motor is encapsulated with an overmold 209. The overmolded stator contains openings through which mounting pins 242 may be inserted for attaching the stator 200 to a base. U.S. Pat. No. 5,672,972 (Viskochil) (incorporated herein by reference) also discloses a spindle motor having an overmolded stator. One drawback with the overmold used in these patents is that it has a different coefficient of linear thermal expansion (“CLTE”) than the corresponding metal parts to which it is attached. This patent also does not teach a method or structure for obtaining predictable and uniform resonance.
U.S. Pat. No. 5,806,169 (Trago) (incorporated herein by reference) discloses a method of fabricating an injection molded motor assembly. However, the motor disclosed in Trago is a step motor, not a high-speed spindle motor, and would not be used in applications such as hard disc drives. The patent does not disclose how to obtain uniform resonance. Thus, a need exists for an improved hard disc drive, hard disc drive components and methods for making the same that overcome the aforementioned problems.